Our overall goal is to establish the basis for a new experimental paradigm for functional magnetic resonance imaging (fMRI) that makes possible the determination of fluctuating brain activity patterns during performance of complex tasks, at rest, or in response to a drug, in quantitative units of absolute cerebral blood flow (CBF). Conventional functional magnetic resonance imaging (fMRI) is based on detection of blood oxygenation level dependent (BOLD) signal modulations. The BOLD signal is a sensitive indicator of underlying physiological changes, but BOLD-fMRI applications are currently limited because the magnitude of the BOLD signal does not provide a reliable quantitative measure of a physiologically meaningful quantity. Arterial spin labeling (ASL) methods provide quantitative measurements of CBF, a well-defined physiological variable. However, sensitive measurement of CBF dynamics remains challenging because of the low signal to noise ratio of the ASL measurement. The key idea of this proposal is a new method to take simultaneous measurements of ASL and BOLD time series, and with an appropriate model of the BOLD response, treat these signals as both being generated from the same underlying time series of CBF fluctuations. The combined data are used to estimate the CBF fluctuations without knowing anything about the underlying drivers of those fluctuations. The proposed new methodology rests on two assumptions: 1) the CBF/CMRO2 coupling ratio for a local region remains constant during the measurement period; and 2) there are no systematic fluctuations of the BOLD signal that are unrelated to CBF fluctuations. Neither assumption is strictly true, so the high risk hal of this proposal is the open question of whether these effects are sufficiently small or can be adequately corrected for the methodology to be robust. We propose to test the feasibility of this method by: Measuring simultaneous ASL and BOLD responses to visual stimuli in healthy human subjects with an experimental paradigm designed to challenge the basic assumptions of the methodology, including variable CBF/CMRO2 coupling, dynamic transitions and BOLD transients (Aim 1); and developing two new analysis techniques to improve the accuracy of the method, one to adapt a recent independent components analysis (ICA) method to use our multi-echo acquisition to identify and remove artifact components in the measured BOLD signals, and the second to improve estimation of the model parameters and deal with a time varying CBF/CMRO2 coupling ratio with a Bayesian approach. The assessment of systematic errors, and the development of robust analysis tools for minimizing their effect, will establish a basis fo widespread application of the new method. This will substantially broaden the possible applications of fMRI, including measurement of brain activity during complex behavior, and quantitative assessments of the effects of development, disease, or drug administration on both the baseline physiological state and stimulus-evoked responses.